One of the limitations of power generation with microbial fuel cells is that the anode chamber must typically be maintained under anaerobic conditions. When oxygen is present in the anode chamber the system is short-circuited because microorganisms oxidize the fuel with the reduction of oxygen rather than electron transfer to the anode. Generally, the anaerobes that would be present on the anode surface under anaerobic conditions are inactivated by oxygen. The need to separate the anaerobic anode chamber from the aerobic cathode chamber creates other electrochemical limitations, such as slow diffusion of protons from the anode to the cathode, which can further limit current production.
Although many potential large-scale applications for producing electrical power with microbial fuel cells have been proposed, at the present stage of development the short-term practical deployment of microbial fuel cells appears limited to localized powering of electronic devices in remote locations (Lovley, D. R. and K. P. Nevin (2008). Electricity production with electricigens. Bioenergy. J. D. Wall, C. S. Harwood and D. A. L. Washington, D.C., ASM Press: 295-306). For example, sediment fuel cells are simple, effective devices that can extract electrons from organic matter naturally present in the aquatic sediments (Reimers, C. E., T. L M, et al. (2001). “Harvesting energy from the marine sediment—water interface.” Environ Sci Technol 35(1): 192-5; Bond, D. R., D. E. Holmes, et al. (2002). “Electrode-reducing microorganisms that harvest energy from marine sediments.” Science 295(5554): 483-5; Tender, L. M., C. E. Reimers, et al. (2002). “Harnessing microbially generated power on the seafloor.” Nat. Biotechnol. 20(8): 821-5; Lowy, D., L. Tender, et al. (2006). “Harvesting energy from the marine sediment-water interface II—Kinetic activity of anode materials.” Biosens Bioelectron 2111: 2058-2063).
FIG. 1 is a schematic of a prior art experimental setup showing coplanar electrodes straddling the marine sediment-seawater interface. Due to distinct differences in chemical composition of the seawater and sediment established by microbial decomposition of organic matter, an open circuit voltage of approximately 0.7 V is observed between the electrodes. According to Reimers et al., Harvesting Energy from the Marine Sediment-Water Interface, Environ. Sci. Technol. 2001, 35, 192-195, the working model for observed power generation involves net oxidation of sediment organic matter by dissolved seawater oxygen, catalyzed by sediment microbes and mediated by one or more secondary electron-transfer mediators. Separation of reactants, necessary to isolate electrode half reactions and allow flow of electrons through the external circuit, is maintained by microbial oxygen depletion in the top layer of sediment.
The anode, which is buried in anoxic sediments, and the cathode, which is suspended in the overlying aerobic water, are both exposed to the environment. There is no need to enclose the anode in a chamber to promote anaerobic conditions at the anode, or to incorporate ion-selective membranes to limit diffusion of oxygen toward the anode, as there is in many other microbial fuel cell applications. This is because the anoxic sediment naturally provides anaerobic conditions at the anode surface. The sediment microbial fuel cell is a static system, eliminating the energy inputs associated with pumping and stirring in many other microbial fuel cell designs. Thus, although the current outputs of sediment fuel cells are low, there is a net energy output sufficient to power electronic monitoring devices (Tender, L. M., S. M. Gray, et al. (2008). “The first demonstration of a microbial fuel cell as a viable power supply: powering a meteorological buoy.” J. Power Sources 179: 571-575). In contrast, most laboratory scale microbial fuel cells serving as prototypes for other microbial fuel cell applications have designs that will consume more energy in long-term application then they produce.
The potential that sediment microbial fuel cells may serve as a long-term power source, extracting energy from a constantly renewing source of organic matter in sediments is very attractive. However, a limitation of sediment microbial fuel cells is the necessity to anchor the anode in anoxic sediments. Microbial fuel cells in which the anode could function in aerobic water would expand the range of aquatic locations in which microbial fuel cells might be used to power electronic devices (Ringeisen, B. R., R. Ray, et al. (2007). “A miniature microbial fuel cell operating with an aerobic anode chamber” Journal of Power Sources 165(2): 591-597), Such ‘aerobic microbial fuel cells’ might have the advantage over traditional batteries in lower costs of materials and presenting a lower explosion hazard during transportation prior to deployment.
Biologically active films (also referred to as “biofilms”) growing on stainless steel anodes were able to produce current in aerobic seawater that was nearly one-third that of a similar anaerobic system (Erable, B. and A. Bergel (2009). “First air-tolerant effective stainless steel microbial anode obtained from a natural marine biofilm.” Bioresource Technology 10(13): 3302-3307). However, the system was not a true microbial fuel cell because the anode was electronically poised at a negative potential, thus net power was not produced. Furthermore, electronically poising the anode could potentially provide electrons to promote removal of oxygen within the biofilm. Current-producing biofilms were not produced if the anode was not electronically poised (Erable and Bergel 2009)
A laboratory microbial fuel cell inoculated with the facultative anaerobe, Shewanella oneidensis, continued to produce power when dissolved oxygen was purposely introduced into the anode chamber (Ringeisen, Ray et al. 2007). In this system a relatively large culture reservoir was maintained under aerobic conditions and continuously cycled through a small anode chamber. Current was produced (6.5 mW/m2 and 13 mA/m2) from a lactate fuel source, despite the presence of oxygen in the anode chamber. Cells did not appreciably attach to the anode and electron transfer between S. oneidensis and the anode was thought to be proceed via electron shuttles (Ringeisen, Ray et al. 2007) which Shewanella species have previously been shown to excrete (Nevin, K. P. and D. R. Lovley (2002). “Mechanisms for Fe(III) oxide reduction in sedimentary environments.” Geomicrobiol. J. 19: 141-159; Marsili, E., D. B. Baron, et al. (2008). “Shewanella secretes flavins that mediate extracellular electron transfer.” PNAS 105: 3968-3973; von Canstein, H., J. Ogawa, et al. (2008). “Secretion of flavins by Shewanella species and their role in extracellular electron transfer.” Appl Environ Microbiol 74: 615-623).
However, the previously described Shewanella-based system would not be directly applicable to powering electronics in aerobic water. It was a complex system requiring continuous pumping to recirculate the anolyte between the anode and the large anolyte reservoir. Therefore, it is likely that the power consumption of the system far exceeds the power output at the anode. Instead of reducing oxygen as the electron acceptor, the catholyte was ferric cynanide, which is a non-renewable, toxic electron acceptor that would not be suitable for long-term field deployments. Columbic efficiencies were low (<6%), when calculated based on the incomplete oxidation of lactate to acetate. The accumulation of two-thirds of the electrons available in lactate in the waste product acetate, meant that the overall efficiency of conversion of fuel to current was even lower.
There are organisms, such as Geobacter species, that can effectively oxidize acetate with electron transfer to electrodes (Bond, D. R., D. E. Holmes, L. M. Tender, and D. R. Lovley. (2002). “Electrode-reducing microorganisms that harvest energy from marine sediments”. Science 295:483-485; Bond, D. R. and D. R. Lovley (2003). “Electricity production by Geobacter sulfurreducens attached to electrodes.” Appl. Environ. Microbiol. 69: 1548-1555). For example, Geobacter sulfurreducens converts acetate to current with columbic efficiencies of over 90% (Nevin, K. P., H. Richter, et al. (2008). “Power output and columbic efficiencies from biofilms of Geobacter sulfurreducens comparable to mixed community microbial fuel cells.” Environ. Microbiol. 10: 2505-2514; Yi, H., K. P. Nevin, et al. (2009). “Selection of a variant of Geobacter sulfurreducens with enhanced capacity for current production in microbial fuel cells.” Biosenors Bioelctron.: (submitted)). Although G. sulfurreducens is considered to be an anaerobe, it is capable of withstanding low levels of oxygen and under these conditions may use oxygen as an electron acceptor to support growth (Lin, W. C., M. V. Coppi, et al. (2004). “Geobacter sulfurreducens can grow with oxygen as a terminal electron acceptor.” Appl Environ Microbiol 70: 2525-8; Leang, C., W. C. Lin, et al. (2010). “Role of cytochrome bd ubiquinol oxidase in scavenging oxygen during low levels of oxygen exposure in Geobacter sulfurreducens.” (submitted))
Traditional microbial fuel cells are two chambered with an anaerobic anode and an aerobic cathode. This design presents numerous engineering problems. Anodes and cathodes must be close to each other in order to facilitate proton transfer and increase power outputs, and proton exchange membranes themselves have limitations. The cathode cannot be anaerobic, as oxygen is needed at the cathode surface to complete the reaction with the electrons and protons. The anodes cannot be aerobic as the bacteria that are capable of producing current do so under anaerobic conditions. Even if a facultative organism or oxygen tolerant anaerobe is used in a system with an oxygen leak, the efficiency of the conversion of fuel to electricity goes down as the organism uses up the oxygen to make an anaerobic environment for itself (Rosenbaum, M., F. Aulenta, et al. (2010). “Cathodes as electron donors for microbial metabolism: which extracellular electron transfer mechanisms are involved?” Bioresource Technology doi:10.1016/j.biortech.2010.07.008).
Proton exchange membranes have been removed in some one chamber microbial fuel cells. However in these cases, the anode is anaerobic and the cathode is aerobic and they are separated by enough space to allow for a gradient of oxygen. Generally, these systems require pumping of liquid from anode to cathode and sparging with air before the liquid reaches the cathode (to allow for the oxygen reaction) or other engineering to allow for the removal the membrane, both of which increase the cost of operating such a system and make the overall system more complicated (Bi, W., Q. Sun, et al. (2009). “The effect of humidity and oxygen partial pressure on degradation of Pt/C catalyst in PEM fuel cell” Electrochimica Acta 54: 1826-1833; Lee, W. J. and D. H. Park (2009). “Electrochemical activiation of nitrate reduction to nitrogen by Ochrobactrum sp. G3-1 using a noncompartmented electrochemical bioreactor.” J. Microbio. Biotechnol. 19: 836-844).
If the membrane could be removed and the anode of a microbial fuel cell made functional in an aerobic environment, the applications of microbial fuel cells could be greatly expanded.
Electrode based microbial fuel cell systems have been widely studied. Some publications that discuss such systems include:
D R Lovley. Microbial fuel cells: novel microbial physiologies and engineering approaches, Current Opinion in Biotechnology Volume 17, Issue 3, June 2006, Pages 327-332; DR Lovley. Bug juice: harvesting electricity with microorganisms, Nature Reviews Microbiology 4, 497-508 (July 2006); D. R. Lovley. The microbe electric: conversion of organic matter to electricity, Current Opinion in Biotechnology Volume 19, Issue 6, December 2008, Pages 564-571; B. E. Logan, B. Hamelers, R. Rozendal, U. Schroder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, and K. Rabaey, Microbial Fuel Cells: Methodology and Technology Environ. Sci. Technol., 2006, 40 (17), pp 5181-5192; and Microbial Fuel Cells, Bruce E. Logan ISBN: 978-0-470-23948-3 Jan. 2008, Wiley.
Microbial fuel cells are primarily run as systems having two chambers to separate the anaerobic anode from the aerobic cathode. In some cases the systems are single-chambered. In these instances the anode is anaerobic and the cathode is usually an air cathode on the outside of the vessel. A paper that describes such a system is Wang, X., Feng, Y. J. and Lee, H. (2008), “Electricity production from beer brewery wastewater using single chamber microbial fuel cell”, Water Science and Technology, 57, 7: 1117-1121.
There is a need for systems and methods that provide single chamber aerobic fuel cells.